Multibit self-reference thermally assisted mram

ABSTRACT

A mechanism is provided for a thermally assisted magnetoresistive random access memory device (TAS-MRAM). A storage layer has an anisotropic axis, in which the storage layer is configured to store a state in off axis positions and on axis positions. The off axis positions are not aligned with the anisotropic axis. A tunnel barrier is disposed on top of the storage layer. A ferromagnetic sense layer is disposed on top of the tunnel barrier.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.14/887,856 entitled “MULTIBIT SELF-REFERENCE THERMALLY ASSISTED MRAM”filed Oct. 20, 2015, which is a divisional of U.S. patent applicationSer. No. 14/583,983, filed Dec. 29, 2014, which claims priority toProvisional Application No. 61/977,236, filed on Apr. 9, 2014, andclaims priority to Provisional Application No. 61/977,243, filed on Apr.9, 2014, all of which are herein incorporated by reference in theirentirety.

BACKGROUND

The present invention relates generally to magnetic memory devices, andmore specifically, to thermally assisted MRAM devices that providemultibit storage in the MRAM device.

Magnetoresistive random access memory (MRAM) is a non-volatile computermemory (NVRAM) technology. Unlike conventional RAM chip technologies,MRAM data is not stored as electric charge or current flows, but bymagnetic storage elements. The elements are formed from twoferromagnetic plates, each of which can hold a magnetic field, separatedby a thin insulating layer. One of the two plates is a reference magnetset to a particular polarity; the other plate's field can be changed tomatch that of an external field to store memory and is termed the “freemagnet” or “free-layer”. This configuration is known as a magnetictunnel junction and is the simplest structure for a MRAM bit. A memorydevice is built from a grid of such “cells.” In some configurations ofMRAM, such as the type further discussed herein, both the reference andfree layers of the magnetic tunnel junctions can be switched using anexternal magnetic field. In some configurations of MRAM, such as thetype further discussed herein, called thermally-assisted MRAM, heat isapplied to the tunnel junction when writing to a bit. In particular, thefree magnet tends to be stable at a normal operating temperature, and itis more difficult to change magnetic polarity of the free magnet at anormal operating temperature. Providing heat to the free magnet mayfacilitate changing of a polarity of the free magnet to program amagnetic state of the free magnet. In particular, in the devicesdescribed herein, in order to thermally write the bit, a magnetic fieldis applied simultaneously with a heating voltage that allows forovercoming the blocking temperature of the antiferromagnetic layer,which exchange biases (providing a pinning direction) with the syntheticantiferromagnet (SAF) storage layer, allowing the storage layermagnetization to be reoriented and re-pinned by exchange bias into thenew position after the device cools.

SUMMARY

According to one embodiment, a thermally assisted magnetoresistiverandom access memory device (TAS-MRAM) is provided. A storage layer hasan anisotropic axis. The storage layer is configured to store a state inoff axis positions and on axis positions. The off axis positions are notaligned with the anisotropic axis. A tunnel barrier is disposed on topof the storage layer. A ferromagnetic sense layer is disposed on top ofthe tunnel barrier.

According to one embodiment, a method is provided for writing data to astate in a thermally assisted magnetoresistive random access memorydevice (TAS-MRAM). The method includes setting a magnetic orientation ofa storage layer to a position for storing the state. The storage layerhas an anisotropic axis, and the position of the magnetic orientationincludes off axis positions and on axis positions relative to theanisotropic axis. The method includes reading the state in the storagelayer to obtain a first resistance value and a second resistance value,and comparing the first resistance value and the second resistance valueto predetermined resistance values. When the first resistance value andthe second resistance value are different from the predeterminedresistance values, the magnetic orientation of the storage layer isreset to the position for storing the state. When the first resistancevalue and the second resistance value match the predetermined resistancevalues, it is recognized that the predetermined resistance values havebeen met to store the state.

According to one embodiment, a method is provided for forming athermally assisted magnetoresistive random access memory device(TAS-MRAM). The method includes providing a storage layer having ananisotropic axis, wherein the storage layer is configured to store astate in off axis positions and on axis positions. The off axispositions are not aligned with the anisotropic axis. The method includesdisposing a tunnel barrier on top of the storage layer and disposing aferromagnetic sense layer on top of the tunnel barrier.

According to one embodiment, a thermally assisted magnetoresistiverandom access memory device (TAS-MRAM) is provided. The device includesa multilayer spacer structure having multiple layers of metalsstructured to inhibit thermal conductivity and structured toelectrically conduct electrical current, an antiferromagnetic pinninglayer disposed on the multilayer spacer structure, and a magnetic tunneljunction disposed on the antiferromagnetic pinning layer. The magnetictunnel junction, the antiferromagnetic pinning layer, and the multilayerspacer structure are patterned to a same pattern.

According to one embodiment, a thermally assisted magnetoresistiverandom access memory device (TAS-MRAM) is provided. The device includesa multilayer spacer structure having multiple layers of metalsstructured to inhibit thermal conductivity and structured toelectrically conduct electrical current, an antiferromagnetic pinninglayer disposed on the multilayer spacer structure, and a magnetic tunneljunction disposed on the antiferromagnetic pinning layer. The magnetictunnel junction includes a storage layer, a non-magnetic tunnel barrierdisposed on the storage layer, and a ferromagnetic sense layer disposedon the non-magnetic tunnel barrier. The ferromagnetic sense layerincludes multiple layers of materials to inhibit thermal conductivitywhile electrically conducting the electrical current.

According to one embodiment, a method of forming a thermally assistedmagnetoresistive random access memory device (TAS-MRAM) is provided. Themethod includes forming a multilayer spacer structure having multiplelayers of metals structured to inhibit thermal conductivity andstructured to electrically conduct electrical current, disposing anantiferromagnetic pinning layer on the multilayer spacer structure, anddisposing a magnetic tunnel junction on the antiferromagnetic pinninglayer. The magnetic tunnel junction includes a storage layer, anon-magnetic tunnel barrier disposed on the storage layer, and aferromagnetic sense layer disposed on the non-magnetic tunnel barrier.The ferromagnetic sense layer includes multiple layers of materials toinhibit thermal conductivity while electrically conducting theelectrical current.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1A illustrates a cross-sectional view of a multibitthermally-assisted magnetoresistive random access memory (TAS-MRAM)device according to an embodiment.

FIG. 1B illustrates a schematic of the anisotropic axis and the possibleorientations of the magnetizations of the layers in the syntheticantiferromagnet (SAF) storage layer according to an embodiment.

FIG. 2A illustrates states (including off axis magnetic orientations)for storing bits in the storage layer according to an embodiment.

FIG. 2B illustrates that the storage layer is composed of a syntheticantiferromagnet (SAF) layer with antiparallel coupling, and that theanisotropic axis of the storage layer is horizontal, i.e., parallel tothe anisotropy axis of the sense layer, according to an embodiment.

FIG. 2C illustrates a typical hysteresis loop corresponding to a SAFstorage layer when the external field is applied along the anisotropyaxis.

FIG. 3A illustrates states (including off axis magnetic orientations)for storing bits in the storage layer according to an embodiment, wherethe anisotropy axis of the storage layer is perpendicular to theanisotropy axis of the sense layer, and where for writing, two differentpolarities (positive and negative directions) of the external appliedmagnetic field are employed.

FIG. 3B illustrates that the anisotropic axis of the storage layer isvertical according to an embodiment, where the anisotropy axis of thestorage layer is in plane, perpendicular to the anisotropy axis of thesense layer.

FIG. 3C illustrates a typical hysteresis loop corresponding to a SAFstorage layer when the external field is applied perpendicular to theanisotropy axis of the storage layer according to an embodiment.

FIG. 4A illustrates states (including off axis magnetic orientations)for storing bits in the storage layer according to an embodiment,wherein the anisotropy axis of the storage layer is perpendicular to theanisotropy axis of the sense layer; for the writing, only one polarity(positive or negative directions, but not both) of the external appliedmagnetic field is employed.

FIG. 4B illustrates that the anisotropic axis of the storage layer isvertical according to an embodiment.

FIG. 4C illustrates a graph of the applied magnetic write field to storeeach state versus the magnetization of the magnetic tunnel junctionaccording to an embodiment.

FIG. 5 illustrates a process for writing the multibit thermally assistedmagnetoresistive random access memory device according to an embodiment.

FIG. 6 illustrates a method for writing data in a state in the multibitthermally assisted magnetoresistive random access memory deviceaccording to an embodiment.

FIG. 7 is a cross-sectional view of a thermally-assistedmagnetoresistive random access memory (TAS-MRAM) device according to anembodiment.

FIG. 8 is a cross-sectional view of a thermally-assistedmagnetoresistive random access memory device according to anotherembodiment.

FIG. 9 illustrates a method of forming the MRAM device(s) according toan embodiment, in which the final operation is not required in FIG. 7,but is utilized in FIG. 8.

FIG. 10 illustrates an example of a computer which can be connected to,operate, and/or include the MRAM device(s) according to an embodiment.

DETAILED DESCRIPTION

Thermally-assisted magnetoresistive random access memory (TAS-MRAM)requires heating of the magnetic tunnel junction stack to a writetemperature (T_(write)) higher than the operating temperature (T_(op))in order to write the device. This is typically done by Joule heatingfrom a bias current that is applied during the write process. The amountof power required to heat the device to T_(write) is strongly dependenton the thermal conductivity between the device and the surroundingstructures and substrate, which are at T_(op)<T_(write).

FIG. 1 illustrates a structure for a multibit self referencethermally-assisted magnetoresistive random access memory (TAS-MRAM)device 100 according to an embodiment.

The structure of the MRAM device 100 includes a magnetic tunnel junction(MTJ) 10. The magnetic tunnel junction 10 may include a storage layer 12with a non-magnetic tunnel barrier 14 disposed on the storage layer 12.The non-magnetic tunnel barrier 14 may be a semiconductor or insulatorwith a high resistance. The magnetic tunnel junction 10 also includes asense ferromagnetic layer 16 disposed on top of the non-magnetic tunnelbarrier 14. The storage layer 12 is made up of a bottom ferromagneticlayer 26 shown with a left magnetic orientation (e.g., left pointingsolid arrow), a non-magnetic spacer 24 disposed on top of the bottomferromagnetic layer 26, and a top ferromagnetic layer 22 shown with aright magnetic orientation (e.g., right pointing open arrow). Thenon-magnetic spacer 24 couples the magnetic orientation of the top andbottom ferromagnetic layers 22 and 26 in opposite magnetic directions inthe storage layer 12. The magnetic orientation of the storage layer 12is based on the direction of the top ferromagnetic layer 22 (i.e., theopen arrow in the storage layer 12). Note that in one case the storagelayer 12 may only include the top ferromagnetic layer 22 with the openarrow in the storage layer 12.

The magnetic orientation of the ferromagnetic sense layer 16 may beflipped to have a left or right magnetic orientation and is shown with adouble open arrow.

The tunnel magnetoresistance (TMR) of the magnetic tunnel junction 10 isbased on the magnetic orientation of the ferromagnetic sense layer 16relative to the magnetic orientation of the top ferromagnetic layer 22of the storage layer 12. When the magnetic orientation of theferromagnetic sense layer 16 is parallel to the magnetic orientation ofthe top ferromagnetic layer 22 (i.e., both arrows point in the samedirection), the resistance of the magnetic tunnel junction 10 (as wellas the TAS-MRAM device 100) is low (i.e., logical 1). When the magneticorientation of the ferromagnetic sense layer 16 is antiparallel to themagnetic orientation of the top ferromagnetic layer 22 (i.e., botharrows point in opposite directions), the resistance of the magnetictunnel junction 10 (as well as the TAS-MRAM device 100) is high (e.g.,logical 0).

TMR is a magnetoresistive effect that occurs in the magnetic tunneljunction (MTJ) 10, which is a component consisting of two ferromagnets(i.e., the ferromagnetic sense layer 16 and the storage layer 12)separated by a thin insulator (i.e., tunnel barrier 14). If theinsulating layer is thin enough (typically a few nanometers), electronscan tunnel from one ferromagnet into the other.

The magnetic tunnel junction 10 (particularly the storage layer 12) isdisposed on top of an antiferromagnetic layer 30. The antiferromagneticlayer 30 is disposed on top of a contact structure 60 (e.g., electrode)that electrically connects the magnetic tunnel junction 10 (MRAM device100) to a voltage source 70 (and/or current source). Theantiferromagnetic layer 30 can also be located below the SAF storagelayer.

The antiferromagnetic layer 30 is an antiferromagnet and may includematerials such as, e.g., IrMn, FeMn, PtMn, etc. The antiferromagneticlayer 30 is composed of two magnetic sublattices. The two magneticsublattices have opposite magnetic orientations (also referred to as amagnetic moments), such that the net magnetic moment of theantiferromagnetic layer 30 is zero. Since antiferromagnets have a smallor no net magnetization, their spin orientation is only weaklyinfluenced by an externally applied magnetic field.

A contact structure 20 (e.g., electrode) is disposed on top of themagnetic tunnel junction 10 (particularly on top of the ferromagneticsense layer 16) connecting the magnetic tunnel junction 10 (MRAM device100) to a wire 40 that connects to the voltage source 70. Voltage and/orcurrent generated by the voltage source 70 are required to heat theTAS-MRAM device 100 to a write temperature (T_(write)).

Now, the write operation for the MRAM device 100 is discussed. Thevoltage source 70 produces a voltage, and the write bias current (i)flows into the wire 40, into the contact structure 20, through the MTJ10, out through the antiferromagnetic layer 30, and out through thecontact structure 60 (back to the voltage source 70). The MTJ 10(particularly the tunnel barrier 14) has a high resistance compared tothe other layers of the MRAM device 100, which causes Joule heating atthe MTJ 10. When the write temperature T_(write) is reached, the heatinghas placed the storage layer 12 in condition to have its magneticorientation flipped and/or changed (e.g., to various positions includingoff axis positions as discussed herein) by a magnetic write fieldapplied by the magnetic write field generating device 80. In otherwords, heating the MTJ 10 to the write temperature T_(write)destabilizes the magnetic orientation of the storage layer 12 so thatthe applied magnetic field can flip and/or rotate the magneticorientation as desired. The magnetic write field generating device 80may be a combination of an (insulated) metal wire connected to a voltagesource (not shown) to generate the magnetic field and/or may be CMOS(complementary metal-oxide-semiconductor) circuitry as understood by oneskilled in the art.

FIG. 1B shows a schematic of the storage layer 12 (from the top down).In FIG. 1B, the anisotropic axis is represented with a horizontal dashedline. The horizontal dashed line illustrates that the anisotropic axisof the storage layer 12 is horizontal. The anisotropic axis is the axisto which magnetic orientation (of the ferromagnetic layers of) thestorage layer 12 wants to align with. In FIG. 1A, the solid arrowrepresenting the magnetic orientation of the bottom ferromagnetic layer26 is pointing left in accord/parallel with the anisotropic axis (i.e.,on axis), while the open arrow representing the magnetic orientation ofthe top ferromagnetic layer 22 is pointing right in accord/parallel withthe anisotropic axis (i.e., on axis).

Note that magnetic anisotropy is the directional dependence of amaterial's magnetic properties. In the absence of an applied magneticfield, a magnetically isotropic material has no preferential directionfor its magnetic moment (i.e., magnetic orientation), while amagnetically anisotropic material will align its moment with one of theeasy axes. An easy axis is an energetically favorable direction ofspontaneous magnetization. The two opposite directions (e.g., left andright magnetic orientations for a horizontal anisotropic axis) along aneasy axis are usually equivalent, and the actual direction ofmagnetization can be along either of them.

According to embodiments, examples of off axis storage of bits (asstates) in the storage layer 12 are provided below. Although themagnetic orientation (solid arrow) of the bottom ferromagnetic layer 26is discussed (for completeness), the TMR (corresponding to the desiredstate) is based on the magnetic interaction of the magnetic orientation(open arrow) of the top ferromagnetic layer 22 and the magneticorientation of the ferromagnetic sense layer 16. FIGS. 2A, 2B, and 2C(generally referred to as FIG. 2) illustrate storing multiple bits inthe MRAM device 100 according to an embodiment. FIG. 2A depicts 4 statesfor storing bits in the storage layer 12. The magnetic orientations ofthe open arrow and the solid arrow can be stored in various off axispositions, and are not limited to the anisotropic axis of the storagelayer 12 (i.e., not limited to being parallel to the anisotropic axis ofthe top and bottom ferromagnets 22 and 26). In this case, FIG. 2B showsthat the anisotropic axis of the storage layer 12 is horizontal.

The states shown for the storage layer 12 in FIG. 2A are state 201,state 202, state 203, and state 204. For explanation purposes it isassumed (in FIGS. 2, 3, and 4) that the anisotropic axis for theferromagnetic sense layer 16 is horizontal and the ferromagnetic senselayer 16 has a magnetic orientation (initially) pointing right for eachof the states 201 through 204. Note that the magnetic orientation of theferromagnetic sense layer 16 may be flipped to the left during thewriting and/or reading process (as desired).

For state 201 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 11 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 7 o'clock). FIG. 2C illustrates a graph 250 ofthe applied magnetic write field (B) (e.g., via the magnetic write fieldgenerating device 80) on the x-axis versus the magnetization of the MTJ10 stack is on the y-axis.

In FIG. 2C, state 201 corresponds to the portion of the curve 255identified at which the applied magnetic (B) field is the most negativeon the x-axis, while the magnetization of the stack (MTJ 10) is mostnegative on the y-axis. The magnetic moment of the ferromagnetic senselayer 16 is pointing right as noted above. State 201 in the curve 255illustrates the applied magnetic write field as applied by the magneticwrite field generating device 80, which is utilized to store the statefor the off axis magnetic orientations shown by the open arrow and solidarrow in the storage layer 12.

For state 202 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is on axis (e.g.,pointing at approximately 3 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is on axis (e.g.,pointing at approximately 9 o'clock). State 202 corresponds to theportion of the curve 255 identified at state 202, which includesnegative and positive applied magnetic (B) fields on the x-axis, whilemagnetization of the stack (MTJ 10) is negative. The magnetic moment ofthe ferromagnetic sense layer 16 is pointing right as noted above. State202 in the curve 255 illustrates the applied magnetic write field asapplied by the magnetic write field generating device 80 that isutilized to store the state 202 for the on axis magnetic orientationsshown by the open arrow and solid arrow in the storage layer 12.

For state 203 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is on axis (e.g.,pointing at approximately 9 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is on axis (e.g.,pointing at approximately 3 o'clock). State 203 corresponds to theportion of the curve 255 identified at state 203, which also includesnegative and positive applied magnetic (B) fields on the x-axis, whilemagnetization of the stack (MTJ 10) is positive. The magnetic moment ofthe ferromagnetic sense layer 16 is pointing right as noted above. State203 in the curve 255 illustrates the applied magnetic write field asapplied by the magnetic write field generating device 80, which isutilized to store the state 203 for the on axis magnetic orientationsshown by the open arrow and solid arrow in the storage layer 12.

For state 204 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 2 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is on axis (e.g.,pointing at approximately 4 o'clock). State 204 corresponds to theportion of the curve 255 identified at state 204, which includespositive applied magnetic (B) fields on the x-axis, while magnetizationof the stack (MTJ 10) is positive. The magnetic moment of theferromagnetic sense layer 16 is pointing right as noted above. State 204in the curve 255 illustrates the applied magnetic write field as appliedby the magnetic write field generating device 80, which is utilized tostore the state 204 for the on axis magnetic orientations shown by theopen arrow and solid arrow in the storage layer 12.

When measuring the magnetic tunnel junction 10 (by measuring theresistance of the MRAM device 100 via voltage source 70 and ammeter 75),each of the 4 states has two measured resistance values: a firstresistance value (kΩ) when the ferromagnetic sense layer 16 is pointingto the right and a second resistance value (kΩ) when the ferromagneticsense layer 16 (or vice versa). Accordingly, state 201 (while themagnetic orientations of the top and bottom ferromagnetic 22 and 26 donot change) has two measured resistance values one higher than theother, state 202 (while the magnetic orientations of the top and bottomferromagnetic 22 and 26 do not change) has two measured resistancevalues one higher than the other resistance value, similarly state 203(while the magnetic orientations of the top and bottom ferromagnetic 22and 26 do not change) has two measured resistance values one higher thanthe other, and likewise state 204 (while the magnetic orientations ofthe top and bottom ferromagnetic 22 and 26 do not change) has twomeasured resistance values one higher than the other.

Typically, the coercive field H_(c) (213, 214) is approximately (˜) 100Oe, the spin-flop field H_(sf) (212, 215) is approximately (˜) 200 Oe,and the saturation field H_(sat) (211, 216) is approximately (˜) 500 Oe.H_(c) is mostly dependent on the difference of magnetizations of the 2layers of the SAF, and H_(sf) and H_(sat) depend on the Ru RKKYantiparallel coupling between the 2 layers of the SAF.

In order to adjust H_(c), H_(sf), and H_(sat), the thickness and thesaturation magnetization of the top and bottom ferromagnetic layers 22and 26 must be adjusted. The saturation magnetization can be tuned bychanging the materials type; materials such as CoFe, CoFeB, and NiFe areuseful. The typical thicknesses can range from 10 to 50 Angstroms (Å)for each ferromagnetic layer 22 and 26.

According to an embodiment, examples of off axis storage of bits (asstates) in the storage layer 12 are provided below. FIGS. 3A, 3B, and 3C(generally referred to as FIG. 3) illustrate storing multiple bits inthe MRAM device 100 according to an embodiment. FIG. 3A depicts 4 statesfor storing bits in the storage layer 12. The magnetic orientations ofthe open arrow and the solid arrow can be stored in various off axispositions, and are not limited to the anisotropic axis of the storagelayer 12 (i.e., not limited to being parallel to the anisotropic axis ofthe top and bottom ferromagnets 22 and 26). In this case, FIG. 3B showsthat the anisotropic axis of the storage layer 12 is vertical (or nothorizontal).

The states shown for the storage layer 12 in FIG. 3A are state 301,state 302, state 303, and state 304. For explanation purposes, it isagain assumed that the anisotropic axis for the ferromagnetic senselayer 16 is horizontal, and the ferromagnetic sense layer 16 (initially)has a magnetic orientation pointing right for each of the states 301through 304. Note that the magnetic orientation of the ferromagneticsense layer 16 may be flipped to the left during the writing and/orreading process (as desired).

For state 301 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 9:45), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 8:50). FIG. 3C illustrates a graph 350 of theapplied magnetic write field (B) (e.g., via the magnetic write fieldgenerating device 80) on the x-axis versus the magnetization (M) of theMTJ 10 stack on the y-axis.

State 301 corresponds to the portion of the curve 355 identified asstate 301 at which the applied magnetic (B) field is the most negativeon the x-axis, while magnetization (M) of the stack (MTJ 10) is mostnegative on the y-axis. The magnetic moment/orientation of theferromagnetic sense layer 16 is pointing right as noted above. State 301in the curve 355 illustrates the applied magnetic write field as appliedby the magnetic write field generating device 80, which is utilized tostore the off axis magnetic orientations shown by the open arrow andsolid arrow in the storage layer 12.

For state 302 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 11 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 7 o'clock). State 302 corresponds to theportion of the curve 355 identified at state 302, which includes anegative applied magnetic (B) fields on the x-axis, while magnetization(M) of the stack (MTJ 10) is negative. The magnetic moment of theferromagnetic sense layer 16 is pointing right as noted above. State 302in the curve 355 illustrates the applied magnetic write field as appliedby the magnetic write field generating device 80 that is utilized tostore the magnetic orientations shown by the open arrow and solid arrowin the storage layer 12.

For state 303 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 2 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is on axis (e.g.,pointing at approximately 5 o'clock). State 303 corresponds to theportion of the curve 355 identified at state 303, which includespositive applied magnetic (B) fields on the x-axis, while magnetization(M) of the stack (MTJ 10) is positive. The magnetic moment of theferromagnetic sense layer 16 is pointing right as noted above. State 303in the curve 355 illustrates the applied magnetic write field as appliedby the magnetic write field generating device 80, which is utilized tostore the on axis magnetic orientations shown by the open arrow andsolid arrow in the storage layer 12.

For state 304 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 2:45), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 3:15). State 304 corresponds to the portion ofthe curve 355 identified at state 304, which includes positive appliedmagnetic (B) fields on the x-axis, while magnetization (M) of the stack(MTJ 10) is positive. The magnetic moment of the ferromagnetic senselayer 16 is pointing right as noted above. State 304 in the curve 355illustrates the applied magnetic write field as applied by the magneticwrite field generating device 80, which is utilized to store the on axismagnetic orientations shown by the open arrow and solid arrow in thestorage layer 12.

When measuring the magnetic tunnel junction 10 (by measuring theresistance of the MRAM device 100 via voltage source 70 and ammeter 75),each of the 4 states has two measured resistance values: a firstresistance value (kΩ) when the ferromagnetic sense layer 16 is pointingto the right and a second resistance value (kΩ) when the ferromagneticsense layer 16. Accordingly, state 301 (while the magnetic orientationsof the top and bottom ferromagnetic 22 and 26 do not change) has twomeasured resistance values one higher than the other, state 302 (whilethe magnetic orientations of the top and bottom ferromagnetic 22 and 26do not change) has two measured resistance values one higher than theother resistance value, similarly state 303 (while the magneticorientations of the top and bottom ferromagnetic 22 and 26 do notchange) has two measured resistance values one higher than the other,and likewise state 304 (while the magnetic orientations of the top andbottom ferromagnetic 22 and 26 do not change) has two measuredresistance values one higher than the other.

In this embodiment, states are obtained by continuously varying theapplied magnetic field form negative to positive values (therefore,using both polarities of applied field). Typical field values for(301-304) are: 301 is approximately (˜) −100 Oe, 302 is approximately(˜) −50 Oe, 303 is approximately (˜) 50 Oe, 304 is approximately (˜) 100Oe.

In order to adjust H_(c), H_(sf), and H_(sat), the thickness and thesaturation magnetization of the top and bottom ferromagnetic layers 22and 26 must be adjusted. The saturation magnetization can be tuned bychanging the materials type; materials such as CoFe, CoFeB, and NiFe areuseful. The typical thicknesses can range from 10-50 Å for eachferromagnetic layer 22 and 26.

According to an embodiment, examples of off axis storage of bits (asstates) in the storage layer 12 are provided below. FIGS. 4A, 4B, and 4C(generally referred to as FIG. 4) illustrate storing multiple bits inthe MRAM device 100 according to an embodiment. FIG. 4A depicts 4 statesfor storing bits in the storage layer 12. The magnetic orientations ofthe open arrow and the solid arrow can be stored in various off axispositions, and are not limited to the anisotropic axis of the storagelayer 12 (i.e., not limited to being parallel to the anisotropic axis ofthe top and bottom ferromagnets 22 and 26). In this case, FIG. 4B showsthat the anisotropic axis of the storage layer 12 is vertical (or nothorizontal). This means the anisotropic axis of the storage layer 12 is90° from the anisotropic axis of the ferromagnetic sense layer 16.

The states shown for the storage layer 12 in FIG. 4A are state 401,state 402, state 403, and state 404. For explanation purposes, it isagain assumed that the anisotropic axis for the ferromagnetic senselayer 16 is horizontal, and the ferromagnetic sense layer 16 (initially)has a magnetic orientation pointing right for each of the states 401through 404. Note that the magnetic orientation of the ferromagneticsense layer 16 may be flipped to the left during the writing and/orreading process (as desired), which changes the resistance of the MTJ10.

For state 401 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 9:05), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 8:55). FIG. 4C illustrates a graph 450 of theapplied magnetic write field (B) (e.g., via the magnetic write fieldgenerating device 80) on the x-axis versus the magnetization (M) of theMTJ 10 stack on the y-axis.

State 401 corresponds to the portion of the curve 455 identified asstate 401 at which the applied magnetic (B) field is the most negativeon the x-axis, while magnetization (M) of the stack (MTJ 10) is mostnegative on the y-axis. The magnetic moment of the ferromagnetic senselayer 16 is pointing right as noted above. State 401 in the curve 455illustrates the applied magnetic write field as applied by the magneticwrite field generating device 80, which is utilized to store the offaxis magnetic orientations shown by the open arrow and solid arrow inthe storage layer 12.

For state 402 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 9:15), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 8:30). State 402 corresponds to the portion ofthe curve 455 identified at state 402, which includes a negative appliedmagnetic (B) fields on the x-axis, while magnetization (M) of the stack(MTJ 10) is negative. The magnetic moment of the ferromagnetic senselayer 16 is pointing right as noted above. State 402 in the curve 455illustrates the applied magnetic write field as applied by the magneticwrite field generating device 80 that is utilized to store the magneticorientations shown by the open arrow and solid arrow in the storagelayer 12.

For state 403 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 10 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is on axis (e.g.,pointing at approximately 8 o'clock). State 403 corresponds to theportion of the curve 455 identified at state 403, which includespositive applied magnetic (B) fields on the x-axis, while magnetization(M) of the stack (MTJ 10) is positive. The magnetic moment of theferromagnetic sense layer 16 is pointing right as noted above. State 403in the curve 455 illustrates the applied magnetic write field as appliedby the magnetic write field generating device 80, which is utilized tostore the on axis magnetic orientations shown by the open arrow andsolid arrow in the storage layer 12.

For state 404 stored in the storage layer 12, the open arrow magneticorientation (of the top ferromagnetic layer 22) is off axis (e.g.,pointing at approximately 11 o'clock), and the solid arrow magneticorientation (of the bottom ferromagnetic layer 26) is off axis (e.g.,pointing at approximately 7:30). State 404 corresponds to the portion ofthe curve 455 identified at state 404, which includes negative appliedmagnetic (B) fields on the x-axis, while magnetization (M) of the stack(MTJ 10) is negative. The magnetic moment of the ferromagnetic senselayer 16 is pointing right as noted above. State 404 in the curve 455illustrates the applied magnetic write field as applied by the magneticwrite field generating device 80, which is utilized to store the on axismagnetic orientations shown by the open arrow and solid arrow in thestorage layer 12.

When measuring the magnetic tunnel junction 10 (by measuring theresistance of the MRAM device 100 via voltage source 70 and ammeter 75),each of the 4 states has two measured resistance values: a firstresistance value (kΩ) when the ferromagnetic sense layer 16 is pointingto the right and a second resistance value (kΩ) when the ferromagneticsense layer 16. Accordingly, state 401 (while the magnetic orientationsof the top and bottom ferromagnetic 22 and 26 do not change) has twomeasured resistance values one higher than the other, state 402 (whilethe magnetic orientations of the top and bottom ferromagnetic 22 and 26do not change) has two measured resistance values one higher than theother resistance value, similarly state 403 (while the magneticorientations of the top and bottom ferromagnetic 22 and 26 do notchange) has two measured resistance values one higher than the other,and likewise state 404 (while the magnetic orientations of the top andbottom ferromagnetic 22 and 26 do not change) has two measuredresistance values one higher than the other.

In this embodiment, states are obtained by continuously varying theapplied magnetic field from small to large values using only a singlepolarity (positive or negative; negative fields are labeled). Typicalfield values for (401-404) are: 401 is greater than (>) −100 Oe, 402 isapproximately (˜) 25 Oe, 403 is approximately (˜) 25 Oe, and 404 isapproximately (˜) 25 Oe.

In order to adjust HL, H_(sf), and H_(sat), the thickness and thesaturation magnetization of the top and bottom ferromagnetic layers 22and 26 must be adjusted. The saturation magnetization can be tuned bychanging the materials type; materials such as CoFe, CoFeB, and NiFe areuseful. The typical thicknesses can range from 10 to 50 Å for eachferromagnetic layer 22 and 26.

Discussion is now directed to writing data to the MRAM device 100 withvariable write time and bit density trade-off. The system makes uses ofan iterative write-check-write process discussed further below.

By varying the (magnetic) write field on the magnetic field line,several distinct resistance states of the MRAM device 100 can beaccessed. Each (predetermined) resistance may have a range/tolerance of,e.g., 5 ohms (1) and still be designated the resistance. As such, eachresistance is encoded as a state in the memory (i.e., in the MTJ 10,which may be considered a memory cell). In one case, the MRAM device 100is configured to encode 3 bits; this requires 8 resistance states, where2³=8. In general, to store N bits in the MRAM device, 2^(N) distinctresistance states are needed. Each bit can be logical high (1) or low(0) where (1) and (0) are distinct values of the resistance of the MRAMdevice 100, which correspond to distinct orientations of the storagelayer 12 (e.g., the top ferromagnetic layer 22 of the storage layer 12in the case of a multilayer storage layer) with respect to theferromagnetic sense layer 16.

With regard to bit density versus write time, since the bit density ofthe MRAM device 100 depends on the ability to incrementally adjust themagnetic orientation of the storage layer 12 (as noted above themagnetic orientation of the storage layer 12 is determined by themagnetic orientation of the top ferromagnetic layer 22 depicted by theopen arrow) with respect to the ferromagnetic sense layer 16, and sincethis relative orientation is a function of the write field magnitude(and possibly the direction of the applied (magnetic) field), andbecause this relative orientation is a result of a magnetostaticinteraction rather than a dynamic one, the bit density for the MRAMdevice 100 is set by the precision with which a control circuit (themagnetic write field generating device 80) can set the magnetic writefield and by the precision with which the read circuit (e.g., thevoltage source 70 and ammeter 75) can resolve the various resistancestates that result from various relative orientations. That is, themaximum bit density is determined primarily by the CMOS drive circuitryin the magnetic write field generating device 80 and readout circuitry(via 70 and 75). This provides for a wide range of tuneability wherebymaximum bit density can be traded off with read and write time tooptimize the same memory cell for various applications.

In particular, a multistep write process may be used for the iterativewrite-read-write process. For example, FIG. 5 illustrates a process 500for writing the multibit TAS MRAM device 100 according to an embodiment.Note that the MRAM device 100, the magnetic write field generatingdevice 80, and readout circuitry (e.g., the voltage source 70 andammeter 75) may each be implemented in a computer 1000 discussed below,as would be understood by one skilled in the art. Reference can be madeto FIGS. 1-4 and 10 (along with FIG. 6 discussed below). In order tothermally write the cells, the different applied fields describedpreviously are applied simultaneously with a heating voltage pulse. Thisheating voltage raises the temperature of the device to greater than theblocking temperature of the antiferromagnetic layer that exchange biasesthe SAF storage layer, which depins the storage layer. This allows thestorage layer magnetization to be reoriented with the applied magneticwrite field. The heating voltage is then reduced back to zero, and thestorage layer re-pinned by exchange bias into the new position after thedevice cools below the blocking temperature of the antiferomagneticpinning layer.

For example, the magnetic write field generating device 80 (e.g., in thecomputer 1000) initially sets the resistance of the MRAM device 100 fora desired/predefined state by adjusting the write field magnitude(and/or direction) which sets the relative magnetic orientations of thestorage layer 12 and ferromagnetic sense layer 16 at block 505. Thedesired state has a desired/predetermined first resistance value and adesired/predetermined second resistance value.

The MRAM device 100 may be read out using a self reference (SR) method(via the computer 1000) to measure a first resistance value and a secondresistance value at block 510. For example, a small voltage (not largeenough to substantially heat the device) is applied by the voltagesource 70 so that current flows through the MRAM device 100, and thefirst resistance value is measured when the magnetic orientation of theferromagnetic sense layer 16 points in the first direction (e.g., pointto the right). Next, while the magnetic orientation of the storage layer12 remains unchanged (e.g., the magnetic orientations of the top andbottom ferromagnetic layer 22 and 26 stay in their same respectivepositions), the magnetic orientation of the ferromagnetic sense layer 16is flipped to the second (opposite) direction (e.g., pointing left), andthe second resistance value is measured. The (magnetic orientation ofthe) storage layer 12 remains fixed because it is pinned by theantiferromagnetic layer 30, and the voltage applied to the MRAM MTJdevice 100 during writing is much smaller than the write voltage, andtherefore is not large enough to heat the device 100 substantially anddepin the storage layer form the antiferomagnetic pinning layer. Theread field is of similar magnitude to the write field.

At block 515, the computer 1000 may compare two measured resistancevalues (e.g., the first resistance value and the second resistancevalue) obtained/measured from the self reference read to thetarget/predetermined two resistance values corresponding to the memorycell's desired state. Each state, such as states 201, 202, 203, 204,states 301, 302, 303, 304, and states 401, 402, 403, 404, has tworesistance values relative to the magnetic orientation of theferromagnetic sense layer 16. As such, when the magnetic orientation isin a first direction (e.g., pointing right) the resistance value of theMRAM device 100 is the first resistance value, and when the magneticorientation is in the second direction (e.g., pointing left) which isopposite the first direction, the resistance value of the MRAM device100 is the second resistance value.

At block 520, responsive to the comparison (by the computer 1000)between the actual first and second resistance values and thedesired/predetermined first and second resistance values, the computer1000 is configured to adjust the write field magnitude (and/ordirection) to adjust actual measured first and second resistance valuesso that the measured first and second resistance values are closer tothe desired first and second resistance values. The graphs 250, 350, and450 show examples of how the write field magnitude can be adjusted toachieve each state. Each state has a range for the magnitude of themagnetic write field that can be applied, so the write field magnitudemay need to be moved in the center of the range for the desired stateand/or applied throughout the entire range.

Blocks 510, 515, and 520 are iteratively repeated until thedesired/predetermined first and second resistance values are obtainedwithin a threshold and/or tolerance at block 525.

This bit density versus write time optimization may be done once for aspecific application of the device, or it may be done dynamically insitu in response to system/device performance demands, energy managementscheme, etc.

Other modes of operation may be utilized for the multibit TAS MRAMdevice 100. For example, it can also be appreciated that instead ofusing this MRAM device 100 as a digital memory device with specificresistance states, the MRAM device 100 may be used in an analog fashion,where a continuum of resistance is available upon programming. In thisrespect, this MRAM device 100 may be especially well suited to otherapplications in analog and mixed signal integrated circuitry, and innovel computing paradigms such as neural-network computing.

Now turning to FIG. 6, a method 600 is provided for writing data in astate in a multibit thermally assisted magnetoresistive random accessmemory device 100 according to an embodiment. In order to thermallywrite the cells, the different applied fields described previously areapplied simultaneously with a heating voltage pulse. This heatingvoltage raises the temperature of the device to greater than theblocking temperature of the antiferromagnetic layer that exchange biasesthe SAF storage layer, which depins the storage layer. This allows thestorage layer magnetization to be reoriented with the applied magneticwrite field. The heating voltage is then reduced back to zero, and thestorage layer re-pinned by exchange bias into the new position after thedevice cools below the blocking temperature of the antiferromagneticpinning layer.

The computer 1000 (via magnetic write field generating device 80) sets amagnetic orientation of the storage layer 12 (e.g., open arrow) to aposition for storing the state (e.g. the desired state may be state 201,but any one of the states 201, 202, 203, 204, states 301, 302, 303, 304,and states 401, 402, 403, 404 (or other states not shown) can beselected) where the storage layer 12 has an anisotropic axis, and wherethe position (e.g., such as the hands on a clock and/or the degrees inthe circle for storage layer 12 shown in FIGS. 2-4) of the magneticorientation includes (any selection from) off axis positions and on axispositions relative to the anisotropic axis at block 605.

The computer 1000 (via voltage source 70 and ammeter 75) reads the statestored in the storage layer 12 to obtain a first resistance value and asecond resistance value at block 610.

At block 615, the computer 1000 compares the first resistance value andthe second resistance value to predetermined resistance values (whichmay be a first desired resistance value and second desired resistancevalue both values of which are known in advance).

At block 620, when the first resistance value and the second resistancevalue are different from the predetermined resistance values (differentfrom the first and second desired resistance values), the computer 1000is configured to reset the magnetic orientation (open arrow) of thestorage layer 12 to the desired position for storing the state. Forexample, the computer 1000 may adjust the magnetic write field (B field)shown in FIGS. 2C, 3C, and 4C in order to achieve the desired state.

At block 625, when the first resistance value and the second resistancevalue match (within a threshold and/or tolerance) the predeterminedresistance values, the computer 1000 recognizes that the predeterminedresistance values have been met to store the desired state (e.g., suchas state 201). Matching the first and second resistance values to thepredetermined resistance values (i.e., the first and second desiredresistance values) verifies that the state (i.e., data) has been storedin the storage layer 12, and thus no further adjustments to themagnitude of the magnetic write field (B field) are needed.

The off axis positions define the desired magnetic orientation of thestorage layer 12 as being aligned off the anisotropic axis of thestorage layer 12. An off axis position for the magnetic orientationmeans that the magnetic orientation of the storage layer 12 is notparallel to the anisotropic axis (e.g., such as states 201, 204 in FIG.2A relative to the anisotropic axis of the storage layer 12 shown inFIG. 2B, states 301-304 in FIG. 3A relative to the anisotropic axisshown in FIG. 3B, and states 401-404 in FIG. 4A relative to theanisotropic axis shown in FIG. 4B).

The magnetic orientation of the storage layer 12 is aligned off theanisotropic axis by a range of 1 to 179 degrees such that the magneticorientation is not parallel to the easy axis. However, if the magneticorientation is off axis by 0 degrees and/or 180 degrees, this would bedefined or the same as being on the anisotropic axis (i.e., parallel tothe anisotropic axis).

A given off axis position storing the desired state has a magneticorientation in the storage layer, and the given off axis position isassociated with a first resistance value (10 kΩ) and a second resistancevalue (30 kΩ). The first resistance value corresponds to and is measured(by the computer 1000) relative to a first magnetic orientation of theferromagnetic sense layer 16 (e.g., right pointing magneticorientation). The second resistance value corresponds to and is measured(by the computer 1000) relative a second magnetic orientation of theferromagnetic sense layer, and the second magnetic orientation (e.g.,left pointing magnetic orientation) is aligned in an opposite positionto the first magnetic orientation of the ferromagnetic sense layer 16.

The state stored is determined by the first resistance value and thesecond resistance value. Each state has two resistance values, whereeach is measured relative to different/opposite pointing magneticorientations of the ferromagnetic sense layer 16.

As noted above, the amount of power required to heat the device toT_(write) is strongly dependent on the thermal conductivity between thedevice and the surrounding structures and substrate, which are atT_(op)<T_(write). For typical devices and structures, the power requiredis undesirably large.

In particular, the TAS-MRAM cell is composed of a magnetic tunneljunction with an antiferromagnetic (AF) pinning layer. This AF layermust be heated to T_(write)>T_(op) in order to write data to the storagelayer (SL) of the TAS-MRAM device.

Embodiments described herein reduce the power required to heat the AFlayer of the device to T_(write) by reducing the thermal conductivitybetween the antiferromagnetic (AF) pinning layer and the surroundingmagnetic layers and contact structures. Embodiments utilize interfacialthermal resistance (sometimes referred to as boundary resistance orKapitza resistance in the literature) in multiple magnetic andnon-magnetic layers within (the sense layer of) the magnetic tunneljunction (MTJ) structure, and below the AF layer in a multilayerthermally resistive spacer structure to reduce the thermal conductivitybetween the AF layer and the surrounding layers and structures.

Empirically and theoretically, thermal resistance is effecteddifferently (and often more strongly) by interfaces than is electricalresistance. This is especially important in metals that are relativelypoor electrical conductors compared to pure Cu, Al, etc., since in thesepoor thermal conductors, thermal conductivity is dominated by phonontransport, whereas in good electrical conductors, thermal conductivityis dominated by electron transport and obeys the Wiedemann-Franz law,displaying a proportionality between electrical and thermalconductivity.

In relatively poorly conducting metals, the relative importance ofphonons in thermal transport is much higher than in materials withrelatively high electrical conductivity. For example, in TaN with even amodest nitrogen concentration, the contribution of electrons and phononsto thermal conductivity is of similar magnitude; for more resistive TaN(commonly used in TAS-MRAM device), phonon thermal conductivity candominate thermal transport. Interfaces, lattice mismatches, amorphouslayers, etc., strongly effect phonon transport; however, if theinterfaces do not present a significant discontinuity in the density ofelectronic conducting states (which is often the case for multilayerconductors), then they will not strongly effect electron transport.Therefore, in relatively electrically resistive metals, in particularmetal nitrides, interfaces can provide a way of significantly reducingthe thermal conductivity of the stack independently from the electricalconductivity, enabling multiple-interface materials/structures to havesmall thermal conductance but similarly large electrical conductance tomaterials/structures with fewer interfaces.

FIG. 7 illustrates a structure for a thermally-assisted magnetoresistiverandom access memory (TAS-MRAM) device 700 according to an embodiment.

The structure of the MRAM device 700 includes a magnetic tunnel junction(MTJ) 710. The magnetic tunnel junction 710 may include a storage layer712 with a non-magnetic tunnel barrier 714 disposed on top of thestorage layer 712. The magnetic tunnel junction 710 also includes aferromagnetic sense layer 716 disposed on top of the non-magnetic tunnelbarrier 714. The non-magnetic tunnel barrier 714 may be a semiconductoror insulator with a high resistance. The storage layer 712 may be aferromagnetic layer.

A contact structure 20 is disposed on the top of magnetic tunneljunction 10 in order to connect the magnetic tunnel junction 710 (MRAMdevice 700) to a first wire 40. The contact structure 20 may be a hardmask to pattern the layers below, when etching is applied.

The storage layer 712 is disposed on top of an antiferromagnetic (AF)pinning layer 750. The antiferromagnetic pinning layer 750 is anantiferromagnet and may include materials such as, e.g., IrMn, FeMn,PtMn, etc. The antiferromagnetic pinning layer 750 may be composed oftwo magnetic sublattices. The two magnetic sublattices have oppositemagnetic orientations (also referred to as magnetic moments), such thatthe net magnetic moment of the antiferromagnetic pinning layer 750 isclose to zero. Since antiferromagnets have a small or no netmagnetization, their magnetic orientation is only weakly influenced byan externally applied magnetic field.

The antiferromagnetic pinning layer 750 is disposed on top of a seedlayer 725. The seed layer 725 is the seed for growing theantiferromagnetic pinning layer 750. Note that the seed layer 725 isoptional, and in one implementation, the seed layer 725 may not bepresent. The seed layer 725, when present, is disposed on top of amultilayer thermally resistive spacer structure 730. When the seed layer725 is not present, the antiferromagnetic pinning layer 750 is disposedon top of the multilayer thermally resistive spacer structure 730. Themultilayer thermally resistive spacer structure 730 is disposed onand/or connected to the second wire 60. The wires 40 and 60 connect theMRAM device 700 to a voltage source 70 (for generating the write biascurrent to heat the MRAM device 700) and ammeter 75 for measuringcurrent. As such, the resistance of the MTJ 710 (i.e., MRAM device 700)can be determined.

The multilayer thermally resistive spacer structure 730 utilizes a largeboundary resistance for heat conduction across interfaces, but therelatively efficient electrical conduction across interfaces, to make anefficient heat barrier for the TAS-MRAM device 700. This also applies tothe multilayer ferromagnetic sense layer 716 observed in FIG. 8.

For example, the electrically conducting multilayer thermally resistivespacer structure 730 has multiple layers of metal (e.g., at least two ormore) shown as different metal layers 730A and 730B. In one case, eachof the individual metal layers 730A and 730B of the multilayer thermallyresistive spacer structure 730 may each be a different metal or metalalloy. Note metal layers 730A and 730B may be a metal (with standardimpurities as understood by one skilled in the art), metal alloys, andcombinations of both. Metals used in the metal layers 730A and 730B mayinclude Ta, TaN, NiCr, NiCrN, and other resistive metals and theirnitrides. These specific materials listed are useful because theyprovide a good base for the other magnetics layers to grow, and becausetheir resistance is high enough to limit the non-interfacialcontribution to thermal conductivity without being so high as to addsignificant series electrical resistance to the device.

In one case, the metal layers 730A and 730B may switch repeatedlybetween two alternating metals/metal alloys, such as Ta (e.g., metallayer 730A) and TaN (e.g., metal layer 730B). Also, the metal layers730A and 730B may alternate among 3, 4, 5, 6, 7, etc., different metallayers in the multilayer thermal resistive contact structure 730, wherethe interface between each metal layer 730A and 730B is between twodifferent metals/metal alloys (i.e., for different conductingmaterials). That is, no two of the same electrically conductingmaterials interface with one another (e.g., are disposed on top of oneanother) in the metal layers 730A and 730B. If metal layer 730A is gold,then metal layer 730B interfacing (i.e., touching metal layer 730A) isnot gold in one case. This principle of not having the same electricallyconducting materials interfacing one another occurs throughout the metallayers 730A and 730B of multilayer thermal resistive contact structure730 (as well as the ferromagnetic sense layer 716 discussed in FIG. 8).

Also, note that the multilayer thermal resistive contact structure 730may optionally include a non-magnetic tunnel barrier 715 (similar to thenon-magnetic tunnel barrier 714). When included, the non-magnetic tunnelbarrier 715 may be a semiconductor or insulator with a high resistancethat provides Joule heating. This second non-magnetic tunnel barrier 715acts as a second source of Joule heat during writing. This is useful insome configurations of TAS-MRAM that require high write temperatures.

As an example, the thickness of each individual metal layer 730A and730B may be between 5 Å (angstroms) to 50 Å thick. The total thicknessof the entire multilayer thermally resistive spacer structure 730 (i.e.,the combined thickness of all of the repeating metal layers 730A and730B) may be between 2 to 50 nm (nanometers).

Further, the metal layers 730A and 730B should have a number ofinterfaces, e.g., two or more (with non-touching metal layers of thesame material). An interface is where one metal layer 730Ameets/contacts a different metal layer 730B (which alternates as seen inFIG. 7). Each interface (of different materials) in the metal layers730A and 730B works to inhibit the escape of heat in the multilayerthermally resistive spacer structure 730 by hindering/restricting theflow of phonons between the interfaces.

A phonon is a collective excitation in a periodic, elastic arrangementof atoms or molecules in condensed matter, such as solids and someliquids. Often referred to as a quasiparticle, the phonon represents anexcited state in the quantum mechanical quantization of the modes ofvibrations of elastic structures of interacting particles. A phonon is aquantum mechanical description of a special type of vibrational motion,in which a lattice uniformly oscillates at the same frequency, and thephonon has both wave-like and particle-like properties. Due to theconnections between atoms in a material, the displacement of one or moreatoms from their equilibrium positions will give rise to a set ofvibration waves propagating through the lattice. The phonons carry heatthrough the lattice vibration. The lattice vibrations work better in asingle material as oppose to the multilayers in the multilayer thermallyresistive spacer structure 730 (and the multilayer ferromagnetic senselayer 716 shown in FIG. 2). The multilayer property reduces thermalconductivity, and the interfaces (between the metal layers 730A and730B) decrease the heat flow by increasing the thermal resistance.

The multiple interfaces between the multiple metal layers 730A and 730B(as well as between metals 716A and 716B in the multilayer ferromagneticsense layer 716 shown in FIG. 8) block the flow of heat in themultilayer contact structure 730 by blocking a flow of phonons betweenthe multiple interfaces. Embodiments are configured to decrease the heatloss and this reduction in heat may vary. For example, embodiments mayhave anywhere from 10-90% reduction in heat loss depending on the typesof materials and number of interfaces used.

In one case, the multilayer thermally resistive spacer structure 730only includes electrically conducting materials of metals and metalalloys. The multilayer thermally resistive spacer structure 730 may bevoid of air vacuums as thermal insulators, void of semiconductors asthermal insulators, void of glass as thermal insulators, and/or void ofdielectrics as thermal insulators.

The multilayer thermal resistive contact structure 730 is patterned tothe same dimensions as the above layers. For example, the multilayerthermal resistive contact structure 730 is patterned to the samedimensions (length and width) and/or size as the magnetic tunneljunction 710, when the contact structure 20 is utilized as a hard maskto etch layers 716, 714, 712, 750, 725, 715, and 730 (all of which areunder the contact structure 20 although not touching) according to thesame pattern.

FIG. 8 illustrates the structure for a thermally-assistedmagnetoresistive random access memory (TAS-MRAM) device 700 according toan embodiment. The elements and discussion in FIG. 7 apply to FIG. 8.Additionally, FIG. 8 illustrates a modification to the ferromagneticsense layer 716. In FIG. 8, the ferromagnetic sense layer 716 hasmultiple adjacent layers of materials that form interfaces similar tothe multilayer thermally resistive spacer structure 730 discussed above.The multilayer ferromagnetic sense layer 716 may include one type ofmetal layer 716A adjacent to another type of metal layer 716B, which arealternatingly stacked to form the ferromagnetic sense layer 716. In onecase, the metal layers 716A are magnetic layers while the metal layers716B are non-magnetic layers. For example, the metal layers 716A may beferromagnetic layers while the metal layers 716B are gold, silver,copper, Ta, TaN, NiCr, and/or NiCrN.

Also, in another case, the metal layers 716A are magnetic layers whilethe metal layers 716B are different magnetic layers. For example, themetal layers 716A may be one ferromagnetic material, and the metallayers 716A may be a different ferromagnetic material. Ferromagneticmaterials include iron, nickel, cobalt and most of their alloys, andmetal layers 716A may be iron and metal layers 716B may be nickel.

The thickness of each individual metal layer 716A and 716B may bebetween 2 Å (angstroms) to 12 Å thick. In one case, the thickness ofeach metal layer 716A and 716B may particularly be 6 Å. Note that themetal layers 716A and 716B in the multilayer ferromagnetic sense layer716 act as a single sense layer. For example, ferromagnetic couplingcause the metal layers 716A and 716B to act as one. Since the layers716A and 716B are thin and in close contact with each other,ferromagnetic exchange interactions, understood by one skilled in theart, couple the layers together into a single magnetic layer thatswitches as a single magnetic moment.

Embodiments described herein in FIGS. 7 and 8 reduce the power (e.g.,voltage and/or current generated by a voltage and/or current source 70)required to heat the TAS-MRAM device 700 to T_(write). Power to generatea write current (via the voltage source 70 to heat the MTJ 710 to thewrite temperature T_(write)) is reduced by reducing the heat thatescapes through the multilayer thermally resistive spacer structure 730(below) and/or the multilayer thermally resistive ferromagnetic senselayer 716 (above). Embodiments presented herein utilize interfacialthermal resistance (sometimes referred to as boundary resistance orKapitza resistance) in the top multilayer ferromagnetic sense layer 716and bottom multilayer thermally resistive spacer structure 730 of theTAS-MRAM device 700 to provide small thermal conductivity.

The multilayer thermally resistive spacer structure 730 significantlyreduces the thermal conductivity (i.e., heat transfer) between the MTJ710 and the wire 60, while allowing electricity to flow freely throughthe multilayer thermally resistive spacer structure 730 to the wire 60.Similarly, the multilayer ferromagnetic sense layer 716 significantlyreduces the thermal conductivity (i.e., heat transfer) out of the top ofthe MTJ 710 to the wire 40, while allowing electricity to flow freelybetween the MTJ 710 and the top wire 40.

Now, the write operation for the MRAM device 700 is discussed. Thevoltage source 70 produces a voltage, and write bias current (i) flowsinto the wire 40, into the contact structure 20, through the MTJ 710(e.g., into the multilayer ferromagnetic sense layer 716 (i.e., thoughmetal layers 716A and 716B), into the antiferromagnetic pinning layer750, into the seed layer 725, out through the multilayer thermallyresistive spacer structure 730 (including the non-magnetic tunnelbarrier 715 along with the metal layers 730A and 730B), and out throughthe bottom wire 60 (back to the voltage source 70). The MTJ 710(particularly the tunnel barrier 714) (and the optional tunnel barrier715) has a high resistance compared to the other layers of the MRAMdevice 700, which causes the Joule heating at the MTJ 710. When thewrite temperature T_(write) is reached, the heating has placed theferromagnetic storage layer 712 in condition to have its magneticorientation flipped or changed by a magnetic field applied by a magneticfield generating device 80. In other words, heating the MTJ 710 to thewrite temperature T_(write) destabilizes the magnetic orientation of theferromagnetic storage layer 712 so that the applied magnetic field canflip the magnetic orientation as desired. In one case, the magneticgenerating device 80 may be a combination of an (insulated) metal wireconnected to a voltage source to generate the magnetic field asunderstood by one skilled in the art. Also, the magnetic generatingdevice 80 may be a CMOS (complementary metal oxide semiconductor)circuit that generates the magnetic filed as understood by one skilledin the art.

In a conventional MRAM (without the multilayer thermally resistivespacer structure 730 and/or without multilayer ferromagnetic sense layer716 with metal layers 716A and 716B but having some other type ofstructure 730 and conventional sense layer), when the write bias currentflows through the MTJ 710, the heat generated at the MTJ 710 (e.g.,particularly at the tunnel barrier 714) is lost because the heat (i.e.,thermal energy) flows away from the MTJ 710 (i.e., flows out through thelayer 20 and into layer 40 in one direction and flows out throughconventional layer 730 and into layer 60 in the other direction if thiswere a convention MRAM). This would require the voltage source 70 togenerate more power to compensate for the heat loss so that the MTJ 710can reach the temperature T_(write).

However, in the embodiments discussed herein, the multilayer thermallyresistive spacer structure 730 and multilayer ferromagnetic sense layer716 (having multiple metal layers 716A and 716B) are designed andpositioned in the MRAM device 700 to trap the heat in the MTJ 710 fromabove and below. Trapping the heat in the MTJ 710 is accomplished by themultilayer thermally resistive spacer structure 730 (via metal layers730A and 730B) blocking heat from flowing out the MTJ 710 at the bottomand by the multilayer thermally resistive ferromagnetic sense layer 716(via metal layers 716A and 716B) blocking heat from flowing out of theMTJ 710 at the top, all while allowing the write bias current (i) toflow through the MRAM device 700.

For example, when the write bias current flows through the MTJ 710, theheat generated by the MTJ 710 (e.g., particularly at the tunnel barrier714) is not lost because as the heat (i.e., thermal energy) tries toflow away above the MTJ 710 (i.e., tries to flow toward the layer 20 andinto layer 40 in one direction), the interfacing metal layers 716A and716B of the multilayer thermally resistive ferromagnetic sense layer 716blocks the flow of heat (as the ferromagnetic sense layer 716 is athermal insulator to block the heat transfer (while allowing electricalcurrent to flow)). Similarly, as the heat (i.e., thermal energy) triesto flow away below the MTJ 710 (i.e., tries to flow toward the layer730, layer 750, and layer 60 in the bottom direction), the multilayerthermally resistive spacer structure 730 blocks the flow of heat (as themultilayer thermally resistive spacer structure 730 is a thermalinsulator to block the heat transfer (while allowing electrical currentto flow)). Accordingly, less heating is needed (because of the reductionin heat loss) which requires the voltage source 70 to generate lesspower (i.e., voltage) to reach the temperature T_(write) for the MTJ710.

FIG. 9 illustrates a method 900 of forming the thermally assistedmagnetoresistive random access memory device (TAS-MRAM) 700 is provided.Reference can be made to FIGS. 7 and 8 (including FIG. 10 below).

At block 905, a multilayer thermally resistive spacer structure 730having multiple layers of metals (e.g., metal layers 730A, 730B, then730A, 730B and so forth) structured to inhibit thermal conductivity,where the multiple layers of metals are structured to electricallyconduct electrical current (e.g., the write bias current (i)).

At block 910, the antiferromagnetic pinning layer 750 is disposed on themultilayer thermally resistive spacer structure 730.

At block 915, the magnetic tunnel junction 710 is disposed on theantiferromagnetic pinning layer 750, and the magnetic tunnel junction710 includes the storage layer 712, the non-magnetic tunnel barrier 714disposed on the storage layer 712, and a ferromagnetic sense layer 716disposed on the non-magnetic tunnel barrier 714.

At block 920, the ferromagnetic sense layer 716 includes multiple layersof materials (e.g., multiple adjacent metal layers 716A and 716B) toinhibit thermal conductivity while electrically conducting theelectrical current.

The multiple layers of metals include multiple interfaces between themultiple layers of metals (e.g., an interface is where metal layer 730Ameets/touches metal layer 730B in the multilayer thermally resistivespacer structure 730), where the multiple layers of metals conduct theelectrical current through the multiple interfaces when voltage (viavoltage source 70) is applied. The multiple interfaces between themultiple layers of metal 730A and 730B block a flow of heat in themultilayer contact structure 730 by blocking a flow of phonons betweenthe multiple interfaces.

The MRAM device 700 includes the hard mask 20 (i.e., contact structure)disposed on top of the magnetic tunnel junction 710. The same pattern ofthe magnetic tunnel junction 710, the antiferromagnetic pinning layer750, and the multilayer spacer structure 730 is patterned based ondimensions of the contact structure 20 (e.g., hard mask) above. Themultilayer spacer structure 730 only includes electrically conductingmaterials as the multiple layers of metals. The multiple layers ofmetals include metal alloys. The multilayer spacer structure 730includes a non-magnetic tunnel barrier 715 in between any two of themultiple layers of metals. The magnetic tunnel junction includes astorage layer, a non-magnetic tunnel barrier disposed on the storagelayer, and a ferromagnetic sense layer disposed on the non-magnetictunnel barrier.

With regard to the multilayer ferromagnetic sense layer 716 discussed inFIG. 8, the multiple layers of materials include at least two differentinterfacing magnetic layers 716A and 716B in a stack to form theferromagnetic sense layer 716. The multiple layers of materials includemultiple interfaces respectively between multiple layers of magneticlayers 716A and 716B. Each meeting/touching location of metal layers716A and 716B is an interface in the multilayer ferromagnetic senselayer 716. The multiple layers of materials include alternating layersof magnetic layers 716A and non-magnetic layers 716B in a stack to formthe ferromagnetic sense layer 716, and the multiple layers of materialsinclude multiple interfaces between the alternating layers of magneticlayers and non-magnetic layers.

FIG. 1000 illustrates an example of a computer 1000 (which may includeone or more MRAM devices 100 and 700) having capabilities, which may beincluded in exemplary embodiments. The MRAM device 100, 700 may beconstructed in a memory array as understood by one skilled in the art(for reading and writing data), and the memory array may be part of thecomputer memory 1020 discussed herein. Various methods, procedures,circuits, elements, and techniques discussed herein may also incorporateand/or utilize the capabilities of the computer 1000. One or more of thecapabilities of the computer 1000 may be utilized to implement, toincorporate, to connect to, and/or to support any element discussedherein (as understood by one skilled in the art) in FIGS. 1-9.

Generally, in terms of hardware architecture, the computer 1000 mayinclude one or more processors 1010, computer readable storage memory1020, and one or more input and/or output (I/O) devices 1070 that arecommunicatively coupled via a local interface (not shown). The localinterface can be, for example but not limited to, one or more buses orother wired or wireless connections, as is known in the art. The localinterface may have additional elements, such as controllers, buffers(caches), drivers, repeaters, and receivers, to enable communications.Further, the local interface may include address, control, and/or dataconnections to enable appropriate communications among theaforementioned components.

The processor 1010 is a hardware device for executing software that canbe stored in the memory 1020. The processor 1010 can be virtually anycustom made or commercially available processor, a central processingunit (CPU), a data signal processor (DSP), or an auxiliary processoramong several processors associated with the computer 1000, and theprocessor 1010 may be a semiconductor based microprocessor (in the formof a microchip) or a microprocessor.

The computer readable memory 1020 can include any one or combination ofvolatile memory elements (e.g., random access memory (RAM), such asdynamic random access memory (DRAM), static random access memory (SRAM),etc.) and nonvolatile memory elements (e.g., ROM, erasable programmableread only memory (EPROM), electronically erasable programmable read onlymemory (EEPROM), programmable read only memory (PROM), tape, compactdisc read only memory (CD-ROM), disk, diskette, cartridge, cassette orthe like, etc.). Moreover, the memory 1020 may incorporate electronic,magnetic, optical, and/or other types of storage media. Note that thememory 1020 can have a distributed architecture, where variouscomponents are situated remote from one another, but can be accessed bythe processor 1010.

The software in the computer readable memory 1020 may include one ormore separate programs, each of which comprises an ordered listing ofexecutable instructions for implementing logical functions. The softwarein the memory 1020 includes a suitable operating system (O/S) 1050,compiler 1040, source code 1030, and one or more applications 1060 ofthe exemplary embodiments. As illustrated, the application 1060comprises numerous functional components for implementing the features,processes, methods, functions, and operations of the exemplaryembodiments. The application 1060 of the computer 1000 may representnumerous applications, agents, software components, modules, interfaces,controllers, etc., as discussed herein but the application 1060 is notmeant to be a limitation.

The operating system 1050 may control the execution of other computerprograms, and provides scheduling, input-output control, file and datamanagement, memory management, and communication control and relatedservices.

The application 1060 may be a source program, executable program (objectcode), script, or any other entity comprising a set of instructions tobe performed. When a source program, then the program is usuallytranslated via a compiler (such as the compiler 1040), assembler,interpreter, or the like, which may or may not be included within thememory 1020, so as to operate properly in connection with the O/S 1050.Furthermore, the application 1060 can be written as (a) an objectoriented programming language, which has classes of data and methods, or(b) a procedure programming language, which has routines, subroutines,and/or functions.

The I/O devices 1070 may include input devices (or peripherals) such as,for example but not limited to, a mouse, keyboard, scanner, microphone,camera, etc. Furthermore, the I/O devices 1070 may also include outputdevices (or peripherals), for example but not limited to, a printer,display, etc. Finally, the I/O devices 1070 may further include devicesthat communicate both inputs and outputs, for instance but not limitedto, a NIC or modulator/demodulator (for accessing remote devices, otherfiles, devices, systems, or a network), a radio frequency (RF) or othertransceiver, a telephonic interface, a bridge, a router, etc. The I/Odevices 1070 also include components for communicating over variousnetworks, such as the Internet or an intranet. The I/O devices 1070 maybe connected to and/or communicate with the processor 1010 utilizingBluetooth connections and cables (via, e.g., Universal Serial Bus (USB)ports, serial ports, parallel ports, FireWire, HDMI (High-DefinitionMultimedia Interface), etc.).

When the computer 1000 is in operation, the processor 1010 is configuredto execute software stored within the memory 1020, to communicate datato and from the memory 1020, and to generally control operations of thecomputer 1000 pursuant to the software. The application 1060 and the O/S1050 are read, in whole or in part, by the processor 1010, perhapsbuffered within the processor 1010, and then executed.

When the application 1060 is implemented in software it should be notedthat the application 1060 can be stored on virtually any computerreadable storage medium for use by or in connection with any computerrelated system or method.

The application 1060 can be embodied in any computer-readable medium foruse by or in connection with an instruction execution system, apparatus,server, or device, such as a computer-based system, processor-containingsystem, or other system that can fetch the instructions from theinstruction execution system, apparatus, or device and execute theinstructions.

In exemplary embodiments, where the application 1060 is implemented inhardware, the application 1060 can be implemented with any one or acombination of the following technologies, which are each well known inthe art: a discrete logic circuit(s) having logic gates for implementinglogic functions upon data signals, an application specific integratedcircuit (ASIC) having appropriate combinational logic gates, aprogrammable gate array(s) (PGA), a field programmable gate array(FPGA), etc.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A method of forming a thermally assistedmagnetoresistive random access memory device (TAS-MRAM), the methodcomprising: forming a multilayer spacer structure having multiple layersof metals structured to inhibit thermal conductivity and structured toelectrically conduct electrical current; disposing an antiferromagneticpinning layer on the multilayer spacer structure; and disposing amagnetic tunnel junction on the antiferromagnetic pinning layer, themagnetic tunnel junction including a storage layer, a non-magnetictunnel barrier disposed on the storage layer, and a ferromagnetic senselayer disposed on the non-magnetic tunnel barrier, wherein theferromagnetic sense layer includes multiple layers of materials toinhibit thermal conductivity while electrically conducting theelectrical current.
 2. The method of claim 1, wherein the multiplelayers of metals include magnetic layers in a stack to form theferromagnetic sense layer.
 3. The method of claim 1, wherein themultiple layers of metals include alternating layers of magnetic layersand non-magnetic layers in a stack to form the ferromagnetic senselayer.
 4. The method of claim 1, wherein, in the multilayer spacerstructure having the multiple layers of metals, a thickness of each ofthe multiple layers of metals ranges from 5 Å to 50 Å.
 5. The method ofclaim 1, wherein, in the multilayer spacer structure having the multiplelayers of metals, a thickness of each of the multiple layers of metalsis 5 Å.
 6. The method of claim 1, wherein the storage layer is disposedon top of the antiferromagnetic pinning layer.
 7. The method of claim 6,wherein the antiferromagnetic pinning layer comprises magneticsublattices having a net magnetic moment of zero.
 8. The method of claim6, wherein the antiferromagnetic pinning layer is disposed on top of aseed layer.
 9. The method of claim 8, wherein the seed layer is disposedon top of the multilayer spacer structure.
 10. The method of claim 1,wherein the multiple layers of metals is selected from the groupconsisting of metals and metal alloys.
 11. The method of claim 1,wherein the multiple layers of metals include alternating layers metalsand metal alloys.
 12. The method of claim 1, wherein the multilayerspacer structure comprises a boundary resistance for heat conductionacross interfaces.
 13. The method of claim 1, wherein the non-magnetictunnel barrier is a semiconductor.
 14. The method of claim 1, whereinthe non-magnetic tunnel barrier is an insulator.
 15. The method of claim1, wherein a contact structure is disposed on the magnetic tunneljunction.
 16. The method of claim 1, wherein the multilayer spacerstructure has at least two layers.
 17. The method of claim 1, whereinthe multiple layers of metals include alternating layers of a predefinednumber of layers.
 18. The method of claim 1, wherein the predefinednumber of layers is
 2. 19. The method of claim 1, wherein the predefinednumber of layers is
 3. 20. The method of claim 1, wherein the predefinednumber of layers is 4.